Such devices have particular application in circumstances where relatively short range line of sight communication is appropriate, and avoid disadvantages inherent in, for example, radio communication or the use of cables. They do have the disadvantage, however, that signal communication is interrupted when there is no direct line of sight between transmitter and receiver. Where relatively short range communication is involved, it has been proposed to overcome the latter disadvantage by providing infra red reflectors or repeaters to enable transmission via an indirect path. This presupposes however that the path or paths of transmission can be planned in advance and does not allow for possible random interruption of signal paths.
One known communication system, for example, makes use of a communication link for polling a number of hand held keypad devices used in gathering opinions from delegates at meetings.
At such a meeting, each delegate is equipped with a hand held keypad. The presenter displays a question on a video screen and the delegates enter their response on their keypad. Each keypad responds to a different address and, as the controlling computer polls the keypad addresses, they each return their delegate's response.
Such a system uses serial bi-directional communications over cables. Most conferences are held in venues which do not provide group response systems so that such a system would be hired in for the duration and installed on a temporary basis. The installation of such a system is labour intensive in view of the requirement for the provision of cabling, and thus a system that avoided the use of cables would provide a significant advance on the state of the art.
WO-A-91 07028 discloses an arrangement wherein a plurality of repeaters are arranged to provide line-of-sight communication between a transmitter and a receiver located within a given area. The repeaters act as passive repeaters of pulses of radiated energy, and are arranged to incorporate a lock-out period wherein each repeater is disabled for a predetermined period of time following its response to a received pulse. Such an arrangement enables line-of-sight propagation of a pulsed signal via a plurality of possible propagation paths, without ambiguity caused by a repeater responding to a pulse that it has already transmitted. In such an arrangement, however, the repeaters, which are intended to be arranged at a fixed spacing, act merely passively to propagate radiation between dedicated transmitters and receivers.
U.S. Pat. No. 5,099,346 describes a system for the propagation of line-of-sight radiation between devices that can receive and transmit in alternative modes in which they act either as originating transmitters, or as passive repeaters of a signal transmitted by another device. In this system, however, for a device to act as a repeater it is necessary for a predetermined signal path to be established so that the repetition of signals is via a defined path through a network. For the device to operate, a relatively complicated communications protocol is therefore required.